Lucas PLC Switches
PLC switches are relatively complex devices; they contain three separate sections that control the ignition, lighting, and dynamo charge rate. Unfortunately, although they are easy enough to dismantle, reassembling one successfully requires patience and dexterity. Here are four detailed online articles on dismantling and rebuilding PLC switches, including exploded views of their construction.
1. An excellent MG website on repairing a PLC:
https://www.mg-cars.org.uk/imgytr/hints&tips/plcswitch.pdf

2. Dorset Austin Seven Club: Repairing a PLC Switch
https://www.da7c.co.uk/technical_torque_articles/repairing_a_plc_switch.htm?

3. Austin Seven Clubs Association: PLC Switch Repair
https://www.austin7.org/TechnicalArticles/PLCSwitch1/PLCSwitch1.html?

4. John Cornforth's beautifully drawn wiring diagrams for the Ruby (Wiring Diagram, Version
4https://www.da7c.co.uk/technical_torque_articles/Ruby_Wiring_v3.pdf?

Until around September, 1931 the ignition switch on an Austin Seven was in effect, the same as that introduced in 1928 on coil-ignition cars, the SM5. The new PLC switch integrated ignition and lighting in one compact unit with single-lever control that automatically switch the charging rate to full when the lights were switched on.
Common versions of the switch include:
PLC2: A four-position switch controlling the ignition, lighting and dynamo charge rates.
PLC5: A three-position switch, marked Off, Side and Head, controlling the ignition and lighting only. This is a drop-in replacement but lacks the ability to run a half charge to the battery in summer.
PLC6: Similar to the PLC5 and likewise controlling only the ignition and lighting.
The screw terminals on the rear of the unit are universally designated as follows:
A: Input from the Ammeter.
IG: Ignition output to the coil and fuses.
T: Tail and side light output.
L: Side light output (turns off when headlamps are selected).
H: Headlight output.
The most common failure occurs in the ignition section, while the lighting and charging switches continue to work normally. A straightforward remedy is to bypass the faulty ignition contacts by fitting a separate switch between the IG and A terminals. This switch can be concealed to provide a crude anti-theft device, although anyone with an ounce of knowledge can hot-wire an Austin Seven in a matter of seconds.
As those who have dismantled an early spade-key PLC switch will know, pressure on the internal ignition contacts is provided by a small spring beneath the metal barrel in which the key turns. The spring pressure is resisted at the top of the switch by an annular metal ring surrounding the keyhole.
If this annular metal ring breaks away completely, the barrel may pop out and the spring fall loose inside the switch. Although the barrel can be pushed back into position, there will then be insufficient pressure to bring the brass contact pads inside the switch together.
If you are fortunate enough to notice the fault before the ring of metal breaks completely, it may be possible to repair the switch without dismantling it. A suitable washer can be fitted over the worn area to retain the barrel and restore the spring pressure. The washer must cover the barrel while still leaving sufficient clearance for the spade key to be inserted and turned.
To effect a repair, clean both the top of the switch and the underside of the washer, preferably with emery paper, to provide sound bonding surfaces. Apply a small quantity of superglue gel, which is much easier to control than the ordinary liquid variety. Do not use too much, unless your intention is to glue the barrel permanently into the switch and convert the ignition key into a purely decorative accessory.
Press and hold the washer firmly in position until the adhesive has set. Once it has dried completely, try the key. With luck, the spring pressure will have been restored, and the ignition switch will work normally once more.
It is, of course, possible to dismantle the switch and repair it properly. Should you attempt this, place the entire assembly inside a large transparent plastic bag before taking it apart. The springs and other small components have been carefully designed to make an immediate and remarkably successful attempt at orbital flight.